Atmospheric Pressure and other State Variables
- Pressure
- Force exerted by random molecular collisions against surfaces
- Units of pressure are Force/Area: Pascals = Newtons/m2
- Common units in U.S. are millibars: 1mb = 100Pa
- Also units of Energy/Volume: (NewtonXm)/m3
- Also units of Momentum Flux: (massXvelocity)/s/m2
- Atmospheric Pressure
- State Variables
- Pressure (p), temperature (T), and volume (V) or density
(
)
- Density equals number of molecules X molecular mass / volume
- Temperature proportional to average molecular kinetic energy
- Pressure proportional to total molecular kinetic energy per volume
- Total number of molecules X average molecular kinetic energy / volume
- Relationships between state variables
- Equation of State
- Relates all three state variables
- Simple form for ideal gases like atmosphere
- Pressure = density X gas constant X temperature
- If any two are known the third can be determined
- Relationship between any two depends on third
- Temperature and density inversely related at same pressure (height)
- Colder air at lower pressure (higher height) can be less dense than warmer air
at higher pressure (lower height)